Micro-electro mechanical system

ABSTRACT

The organic MEMS according to the present invention comprises a polymeric substrate comprising a substrate surface including a first region and a second region. A polymer coating is applied to the first region to provide a coating surface that is spaced apart from the substrate surface. A terminal is disposed on the second region. A metallic trace is affixed to the coating surface such that the metallic trace forms a flexible extension over the second region. The extension has a rest position where the extension is spaced apart from the terminal, and a flexed position where the extension is disposed towards the terminal. An actuator is used to provide an electric field to deflect the extension from the rest position to the flexed position. By changing the spacing between the extension and the terminal, it is possible to change the electrical condition provided by the MEMS.

FIELD OF THE INVENTION

[0001] The present invention relates to an organic micro-electromechanical system that can be fabricated within or on the surface of anorganic Printed Wiring Board (PWB) utilizing high density interconnect(HDI) substrate technology.

BACKGROUND OF THE INVENTION

[0002] Smaller and more complex electronic devices require smallerswitches. Current solid-state switches are not ideal, because theyexhibit a finite leakage that precludes a complete “off” state. On theother hand, current mechanical and electromechanical switches are bulkyand consume a large amount of power. Micro electromechanical systems(MEMS) have been reported to address the drawbacks of the prior art. SeeU.S. Pat. No. 5,051,643 to Dworsky and Chason, 1991; and U.S. Pat. No.5,578,976 to Yao, 1996. However, the above-referenced MEMS arefabricated from crystalline silicon or ceramic silicon dioxide thatrequire fabrication methods (e.g., reactive ion etching, vapordeposition, etc.) that are not compatible with printed wiring board(PWB) fabrication. Therefore, MEMS made by this technology must be madeseparately, then incorporated into printed wiring boards.

[0003] Moreover, crystalline silicon or silicon dioxide ceramic tends tobe stiff. Accordingly, these materials are only useful for makingswitches that have relatively small gaps (e.g., ≦1 micron), not switcheshaving relatively large gaps (e.g., >1 micron), and these switchesrequire a higher activation voltage than switches having a lower elasticmodulus. It would be desirable to form MEMS switches that are not basedon crystalline silicon or ceramic silicon dioxide.

[0004] The organic MEMS according to the present invention can befabricated during fabrication of the printed wiring board (PWB), and areuseful for switches having a wide range of gaps (about 1-25 microns).The organic MEMS comprises a polymeric substrate comprising a substratesurface including a first region and a second region. A polymer coatingis applied to the first region to provide a coating surface that isspaced apart from the substrate surface. A terminal is disposed on thesecond region. A metallic trace is affixed to the coating such that themetallic trace forms a flexible extension over the second region. Theextension has a rest position where the extension is spaced apart fromthe terminal, and a flexed position where the extension is disposedtowards the terminal. An actuator is used to provide an electric fieldto deflect the extension from the rest position to the flexed position.By changing the spacing between the extension and the terminal, it ispossible to change the electrical condition provided by the organicMEMS. Because, the extension is not supported by a material such ascrystalline silicon or silicon dioxide ceramic, the organic MEMS iscompatible with PWB fabrication, and provides a wider range ofdeflection gaps at a lower activation voltage.

[0005] The extension and the terminal need not contact each other tochange the electrical condition provided by the organic MEMS. Bychanging the distance between the extension and the terminal, a variablecapacitor is formed, wherein in the rest position, the MEMS has onecapacitance, while in the flexed position, the MEMS has anothercapacitance. The organic MEMS and the method of fabrication arecompatible with PWB fabrication and are used to make PWB embeddedswitches and capacitors.

[0006] The present invention is also directed to a method of forming theorganic MEMS comprising depositing an electrode at the second region ofa polymeric substrate comprising a substrate surface including a firstregion and a second region, then applying a photopolymer coating overboth regions and the electrode. The photopolymer is selectivelyirradiated in the first region to form an insoluble coating in the firstregion, while a soluble coating remains in the second region. A metaltrace is fixed to the coating such that a flexible extension overlapsthe electrode. The soluble coating is removed to expose the electrodesuch that the electrode is spaced apart from the extension.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIGS. 1A-J show cross-sectional views that illustrate the stepsfor making two MEMS embodiments having a cantilever extension;

[0008]FIG. 2A shows an organic MEMS in which the metal trace defines adiaphragm extension;

[0009]FIG. 2B shows a top view the MEMS of FIG. 2A in which thediaphragm extension has been removed to expose the dielectric surface;and

[0010] FIGS. 2C-D show cross-sectional views of the MEMS of FIGS. 2A and2B across line S-S.

DETAILED DESCRIPTION OF THE DRAWINGS

[0011] In FIG. 1, a polymer substrate 12 with a metal layer 14 onsubstrate surface 16 (FIG. 1A) is treated to form electrodes 18 and 20in second region 24, such that metal layer 14 remains in first region 22(FIG. 1B). Formation of electrodes 18 and 20 can be accomplished usingmetal print and etch processes widely known in the printed wiring boardindustry. Photopolymer 26 is applied over both regions of the substrate,including metal layer 14 and electrodes 18 and 20, and selectivelyirradiated in region 22 with radiation so that the photopolymer becomesinsoluble in that region (FIG. 1C). A metal trace 28 is fabricated onphotopolymer 26 over both first and second regions 22 and 24,respectively (FIG. 1D). The metal trace 28 can be formed, for example,by first laminating a metal foil (such as copper) to the photopolymerlayer using low temperature lamination, and then printing and etchingthe metal to form the metal trace 28. To form MEMS 36, photopolymer 26in region 24 that was not exposed to the radiation is removed bydissolving in a suitable solvent. An insoluble coating 30 over firstregion 22 on which metal trace 28 is fixed on coating surface 32, and anextension 34 over second region 24, set apart from electrodes 18 and 20(FIG. 1E), is thus formed.

[0012] For MEMS 36, electrode 18 is shown to be thicker than electrode20. In one embodiment, the shorter height of electrode 20 can beachieved, for example, by selectively thinning the electrode metal usingcontrolled depth etching processes known in the printed wiring boardindustry. Accordingly, when MEMS 36 is a switch, electrode 20 is theactuator and electrode 18 is the terminal. As an electric field iscreated at electrode 20, extension 34 is drawn towards electrode 20until extension 34 makes contact with electrode 18 in order to completea circuit. Alternatively, when MEMS 36 is a variable capacitor,electrode 18 is an actuator. As an electric field is created atelectrode 18, extension 34 is drawn towards electrode 18 until extension34 makes contact with electrode 18. As the extension 34 is deflectedfrom a rest state to a flexed state, the gap between extension 34 andelectrode 20 changes. The different gaps produce different capacitancestates between extension 34 and electrode 20. Those skilled in the artwould recognize alternative embodiments, such as, for example, having athicker electrode 20 than electrode 18 (not shown).

[0013] In FIG. 1F, only electrode 18 is formed in the second region 24on surface 16. As described above, photopolymer 26 is applied, thenselectively irradiated in first region 22 (FIG. 1G). Metal trace 28 isfabricated on photopolymer 26 over both the first and second regions 22and 24, respectively (FIG. 1H). A polymer backing 38 can be formed overmetal trace 28 (FIG. 11). MEMS 40 is formed when soluble photopolymer 26is selectively removed to form insoluble coating 30, on which metaltrace 28 is fixed on coating surface 32 and forms an extension 34 oversecond region 24, set apart from electrode 18 (FIG. 1J) In thisembodiment, electrode 18 is both the actuator and the terminal.

[0014] Examples of polymer substrate encompass any PWB material, such aspolymers and reinforced polymer composites. Common resin vary from epoxyto Teflon. Common reinforcing materials include woven or non-woven glassfabrics or organic fibers (e.g., aromatic polyamide polymer-aramidpaper). Particular materials include epoxy, polyamide, polyimide,modified epoxy, BT epoxy, cyanate ester, PTFE, E-glass, S-glass, aramidpaper, FR-4, modified epoxy-aramid, modified epoxy-SI-glass, CE-E-glassand PTFE (Gore).

[0015] Any polymer can form the coating for the MEMS according to thepresent invention, including photopolymers. In one embodiment, thepolymer can be a photopolymer such as an HDI photoimageable dielectric.Examples of such photopolymers, included for example only and not aslimitations on the scope of the present invention, can be Probelec™ 7081(Ciba Specialty Chemicals) or ViaLux™ 81 (DuPont) HDI photoimageabledielectric. After the soluble polymer is selectively removed, theinsoluble coating may be cured.

[0016] The conductive components of the MEMS, such as the electrodes andmetal trace are fabricated by known methods. Examples includeelectroless or electroplate deposition of copper, gold, aluminum,platinum, nickel, silver, chrome, palladium, tin, bismuth, indium, lead,and alloys thereof, such as gold-palladium. The metal can also belaminated on the polymer substrate. Examples include electroless orelectroplate deposition of copper, gold, aluminum, platinum, nickel,silver, chrome, palladium, tin, bismuth, indium, lead, and alloysthereof, such as gold-palladium. To define the conductive components,the plated or laminated metals are pattern etched by wet or dry etchmethods.

[0017] As shown in FIG. 11, in addition to the metal trace, theextension described herein has an optional backing that is not made fromcrystalline silicon or ceramic silicon dioxide. Such backings are madefrom organic dielectric materials, such as, for example, epoxies,polyacrylates or polyimides. For example, in one embodiment presented asan example and not to limit the scope of the present invention, thebacking material can be epoxy polyacrylate. Photoimageable dielectricsmay also be used as backing materials. Extensions can be made fromCu-clad polyimide, epoxy resin coated foil (RCF), or copper, forexample. Use of just the metal or a metal with a polymer backing,provides a switch that requires less activation voltage, and can be usedto close larger gaps. The extension described herein may take manyforms, such as a simply supported beam, a cantilever beam, plate ordiaphragm.

[0018]FIG. 2A shows MEMS 42 with a metal trace 28 that forms a diaphragmon coating 30 and over polymer substrate 12. FIG. 2B shows a top view ofMEMS 42 in which the metal trace is removed to reveal substrate surface16 in second region 24, with electrode 20 forming a concentric ringaround dielectric layer 46. FIGS. 2C-D are cross-sectional views of MEMS42 across line S-S, showing polymeric substrate 12 with surface 16having first region 22 and second region 24. Metal trace 28 is fixed onthe insoluble coating 30, and forms an extension 34 over second region24. Electrode 20 and electrode 18 are disposed in second region 24, onsurface 16. As shown in MEMS 42, electrode 18 could have a dielectriclayer 46 on an electrode surface 44. The dielectric layer could beceramic, polymer, oxide or a polymer-inorganic material. FIG. 2C showsMEMS 42 in a rest position where extension 34 is set apart fromelectrode 18. FIG. 2D shows MEMS 42 in a flexed position where electrode20, as the actuator, has deflected extension 34 to contact dielectricceramic layer 46 on electrode 18.

[0019] One advantage of the organic MEMS and process for forming theorganic MEMS according to the present invention, is the compatibility ofthe MEMS and PWB fabrication process. Such MEMS can be embedded in anHDI layer, fabricated on the PWB surface, or over a metal or dielectriclayer on the PWB or any substrate surface. As part of the HDIfabrication, the organic MEMS is used as an electronic circuit elementin connecting resistors, capacitors and inductors embedded in thesubstrate, or placed on the substrate providing for optimal circuitperformance, reducing inductance by reducing the length of the signalpath between an IC I/O and the electronic circuit element, andminimizing assembly costs.

[0020] While the present invention has been described in terms ofparticular embodiments, it is apparent that one skilled in the art canadopt other forms without departing from the scope and spirit of thisinvention. Accordingly, the scope of the invention is limited only bythe literal and equivalent scope of the claims that follow. In addition,any art cited herein is incorporated by reference.

We claim:
 1. A method for manufacturing an electronic circuit elementcomprising: providing a substrate comprising a surface including a firstregion and a second region, fabricating an electrode on the secondregion; applying a film of a photosensitive polymeric material on thefirst region, the second region and the electrode, the photosensitivepolymeric material having a soluble state prior to irradiation and aninsoluble state after irradiation; selectively irradiating the film toform an insoluble coating on the first region, and a soluble coating onthe second region and the electrode; fabricating a metallic trace on thefilm, the metallic trace affixed to the insoluble coating and forming anextension on the soluble coating that overlaps the electrode; andremoving the soluble coating from the second region to expose theterminal, such that the electrode is spaced apart from the extension. 2.The method of claim 1 wherein the substrate is selected from the groupconsisting of polymer, ceramic, silicon, gallium arsenide,semiconductor, metal, and glass.
 3. The method of claim 1 wherein thefilm is formed of a photoimageable polymer.
 4. The method of claim 1wherein the film is selected from the group consisting of aphotoimageable polyimide, epoxy, and acrylate.
 5. The method of claim 1wherein a second electrode is formed on the second region prior toapplying the film.
 6. The method of claim 1 wherein the electrode isformed by plating a metal layer on the substrate and pattern etching themetal layer to define the electrode.
 7. The method of claim 6 whereinthe plating method is selected from the group consisting ofelectroplating and electroless plating.
 8. The method of claim 6 whereinthe metal layer is formed of a metal selected from the group consistingof copper, aluminum, platinum, gold, nickel, silver, chrome, palladium,tin, bismuth, indium, lead, gold-palladium, and alloys thereof.
 9. Themethod of claim 6 wherein a second electrode is defined by patternetching the metal layer.
 10. The method of claim 1 wherein the electrodeis formed by laminating a metal layer on the substrate and patternetching the metal layer to define the electrode.
 11. The method of claim10 wherein the metal layer is formed of a metal selected from the groupconsisting of copper, aluminum, platinum, gold, nickel, silver, chrome,palladium, tin, bismuth, indium, lead, gold-palladium, and alloysthereof.
 12. The method of claim 10 wherein a second electrode isdefined by pattern etching the metal layer.
 13. The method of claim 1wherein the insoluble coating is cured after the soluble coating isremoved.
 14. The method of claim 1 wherein the metallic trace isfabricated by laminating a metal layer on the film and pattern etchingthe metal layer to define the metallic trace.
 15. The method of claim 14wherein the metallic trace is formed of metal selected from the groupconsisting of copper, aluminum, platinum, gold, nickel, silver, chrome,palladium, tin, bismuth, indium, lead, gold-palladium, and alloysthereof.
 16. The method of claim 1 wherein the metallic trace isfabricated by plating a metal layer on the film and pattern etching themetal layer to define the metallic trace.
 17. The method of claim 16wherein the plating method is selected from the group consisting ofelectroplating and electroless plating.
 18. The method of claim 16wherein the metal layer is formed of a metal selected from the groupconsisting of copper, aluminum, platinum, gold, nickel, silver, chrome,palladium, tin, bismuth, indium, lead, gold-palladium, and alloysthereof.
 19. The method of claim 1 wherein a metal layer is formed inthe first region as the electrode is formed on the second region, andthe film is applied over the metal layer.
 20. An electronic circuitelement comprising: a substrate comprising a substrate surface includinga first region and a second region; a polymer coating applied to thefirst region, the polymer coating including a coating surface spacedapart from the substrate surface; a terminal disposed on the secondregion; a metallic trace affixed to the coating surface such that themetallic trace forms an extension over the second region, whereby theextension has a rest position where the extension is spaced apart fromthe terminal, and a flexed position where the extension is disposedtowards the terminal; and an actuator disposed on the second regioncapable of creating an electric field effective to flex the extensionfrom the rest position to the flexed position.
 21. The electroniccircuit element of claim 20 wherein the substrate is selected from thegroup consisting of polymer, ceramic, silicon, gallium arsenide,semiconductor, metal, and glass.
 22. The electronic circuit element ofclaim 20 wherein the polymer coating is formed of a photopolymer. 23.The electronic circuit element of claim 20 wherein the polymer coatingis formed of a material selected from the group consisting of polyimideand epoxy.
 24. The electronic circuit element of claim 20 wherein thepolymer coating is formed of photoimageable polymer.
 25. The electroniccircuit element of claim 20 wherein the substrate is a reinforcedpolymer composite.
 26. The electronic circuit element of claim 20wherein a metal layer is interposed between the substrate and thepolymer coating.
 27. The electronic circuit element of claim 20 whereinthe extension has a free end that is remote from a fixed end on thepolymer coating.
 28. The electronic circuit element of claim 20 whereinthe extension has a free end that is remote from a fixed end on thepolymer coating and is simply supported.
 29. The electronic circuitelement of claim 20 wherein the polymer coating further comprises fixedfirst and second edges disposed about the second region, and theextension bridges the second region between the fixed first and secondedges.
 30. The electronic circuit element of claim 29 wherein the firstand second edges surround the second region, and the extension forms adiaphragm over the second region.
 31. The electronic circuit element ofclaim 29 wherein the first and second edges surround the second region,and the extension forms a plate over the second region.
 32. Theelectronic circuit element of claim 20 wherein the polymer coatingfurther comprises a fixed first edge and a simply supported second edgedisposed about the second region, and the extension bridges the secondregion between the fixed first edge and the simply supported secondedge.
 33. The electronic circuit element of claim 20 wherein theterminal is also the actuator.
 34. The electronic circuit element ofclaim 20 wherein the terminal is distinct from the actuator.
 35. Theelectronic circuit element of claim 20 wherein the extension in theflexed position makes contact with the terminal.
 36. The electroniccircuit element of claim 20 wherein there is a gap between the extensionand the terminal when the extension is in the flexed position.
 37. Theelectronic circuit element of claim 20 wherein the extension forms aplate over the second region.
 38. The electronic circuit element ofclaim 20 wherein the extension forms a diaphragm over the second region.39. The electronic circuit element of claim 20 wherein the extensionforms a cantilever having a free end over the second region.
 40. Aprinted wiring board having a switch, the switch comprising: a substratecomprising a substrate surface including a first region and a secondregion; a polymer coating applied to the first region, the polymercoating including a coating surface spaced apart from the substratesurface; a terminal disposed on the second region; a metallic traceaffixed to the coating surface such that the metallic trace forms aextension over the second region, the extension having a rest positionwherein the extension is spaced apart from the terminal and a flexedposition wherein the extension is disposed towards the terminal; and anactuator disposed on the second region capable of creating an electricfield effective to flex the extension from the rest position to theflexed position.
 41. The printed wiring board of claim 40 wherein theextension contacts the terminal in the flexed position.
 42. The printedwiring board of claim 40 wherein there is a gap between the extensionand the terminal when the extension is in the flexed position.
 43. Theprinted wiring board of claim 40 wherein the extension forms a plateover the second region.
 44. The printed wiring board of claim 40 whereinthe extension forms a diaphragm over the second region.
 45. The printedwiring board of claim 40 wherein the extension forms a cantilever havinga free end over the second region.
 45. The printed wiring board of claim40 wherein the terminal is an electrode having a first gap from theextension in the resting position and the actuator is a second electrodehaving a second gap from the extension in the resting position, suchthat when the extension is in the flexed position, the extension makescontact with the terminal and there is a gap between the terminal andthe actuator.
 46. The printed wiring board of claim 40 wherein theextension is supported by an organic polymer backing.
 47. A printedwiring board having a variable capacitor, the variable capacitorcomprising: a substrate comprising a substrate surface including a firstregion and a second region; a polymer coating applied to the firstregion, the polymer coating including a coating surface spaced apartfrom the substrate surface; a terminal disposed on the second region; ametallic trace affixed to the coating surface such that the metallictrace forms a extension over the second region, the extension having arest position wherein the extension is spaced apart from the terminaland a flexed position wherein the extension is disposed towards theterminal; and an actuator disposed on the second region capable ofcreating an electric field effective to flex the extension from the restposition to the flexed position.
 48. The printed wiring board of claim47 wherein the extension is spaced apart from the terminal by a firstgap in the rest position and the extension is spaced apart from theterminal by a second gap less than the first gap in the flexed position.49. The printed wiring board of claim 47 further comprising a dielectriclayer disposed between the terminal and the extension, such that thereis a gap between the extension and the dielectric layer when theextension is in the rest position and the extension contacts thedielectric layer when the extension is in the flexed position.
 50. Theprinted wiring board of claim 49 wherein the dielectric layer isselected from the group consisting of ceramic, polymer, oxide, and apolymer-inorganic material
 51. The printed wiring board of claim 47wherein the substrate is selected from the group consisting of polymer,ceramic, silicon, gallium arsenide, semiconductor, metal, and glass.